This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. There is growing concern regarding the impact of metals and metalloids on organisms in the environment and ultimately on humans. Some elements such as mercury are notorious as toxic elements. In contrast the role of selenium is more ambiguous, since though toxic at higher levels, it is an essential element for mammals and many other organisms at trace concentrations. However, with each of these elements the chemical speciation critically affects the properties of the element, including its mobility in groundwater, bioavailability and toxicity especially to higher organisms such as birds, fish and humans. Synchrotron x-ray techniques, such as are described in this proposal, provide speciation and localization information for metals and metalloids such as Se or Hg. Such studies can be carried out in materials as diverse as lakebed sediments or tissues of fish without intrusive methods that often perturb the chemistry of the matrix and therefore the speciation. Thus, the synchrotron provides unique tools for studying complex samples of environmental interest. Furthermore, at SSRL we have achieved such measurements with high sensitivity making these methods ideal for studying samples at ?natural? concentrations. In this program we propose two contrasting lines of research. One is a holistic approach to study the effects of selenium in a mine-impacted site, the results of which should have widespread implications for selenium in aquatic coldwater environments. The second is a laboratory-based study using larval zebrafish to investigate in detail the localization of impact of mercury, selenium and other elements. Such a study has ramifications not only for environmental toxicology but also for human toxicology, as zebrafish have been used as a model for human development.